Art of fabricating electron tubes



April 1961 M. B. SHRADER E'l'AL 2,980,984

ART OF FABRICATING ELECTRON TUBES Filed Aug 19, 1957 2 Sheets-Sheet lawn rams ]\/[ER'RALD B. SHRADER Munms R. WEINEARTEN FRED E. BLDEK April1961 M. B. SHRADER ETAL 2,980,984

ART OF FABRICATING ELECTRON TUBES Filed Aug. 19, 1957 2 Sheets-Sheet 2 i4 i "la-an 0 as k m i/vrmi MERBALD B. SHBA'DER Munms R.WEINEARTN BY FRED55mm United States Patent ART OF FABRICATING ELECTRON TUBES Merrald B.Shrader, Mount Joy, and Fred G. Block and Morris R. Weingarten,Lancaster, Pa., assignors to Radio Corporation of America, a corporationof Delaware Filed Aug. '19, 1957, Ser. No. 678,790

6 Claims. c1. 29-2544 This invention relates to the art of fabricatingelection tubes of the type having a plurality of thin aperturedelectrodes wherein the apertures of one of the electrodes are to bealigned in a desired direction with the apertures of another of theelectrodes. I

In the design and tabricationof electron tubes having a plurality ofaperturedelectrodes, particularly those which are to be operated at highpower levels, it is important that the electrodes be mounted andmaintained so that their structural elements are in accuratedesiredalignment with each other along the cathode-anode electronstreampath. For example, in the case of a tube having a control grid and ascreen grid, if the conductive elements of the screen grid are notperfectly shielded by, or aligned with, the conductive elements of thecontrol grid, electron impingement upon the screen grid will result.Because of this impingement, some electrons emitted by the cathode failto reach the anode and are thus not utilized to produce output power.This means not only an inefficient use of the electrons available fromemission, but also an inefiicient use of input power. Such impingementalso means screen grid current, which causes an undesirable heating ofthe tube and consequently a heat dissipation problem. Both of theseplace a distinct.

limitation on the operational ratings of the tube.

In spite of this consideration, plural grid electron tubes fabricatedaccording to the prior art have consisted of grid units which wereseparately fabricated and then mounted together and roughly aligned toform the device. Such fabrication involves many operations, any one ofwhich if improperly performed could impair the quality of the assembledtube. For example, ina tube with cylindrical gridsthe control grid and.screen. grid.

units. must be fabricated with identical configurations but withthescreen grid somewhat larger. Even assuming. that a pair ofgrids couldbeso constructed according.

bly fixed to' maintain this alignment. In the prior art. it has been thepractice to manually align each pair ofgrid wires separately. This hasbeen a time consuming and. costly procedure. equipment, such as'shadowgraph apparatus and the like,

such manual alignment has actually been a matter of' a' finite degreeofmisalignment.

It is therefore an object of our invention to' provide an iinprovedmethod of fabricating an assembly of apertured electrodes wherein'theapertures of'eaeh'el'ectrode Even with the help of precision 'ice are insubstantially perfect alignment with the corresponding apertures of theother of such electrodes.

Briefly, according to our invention, two or more thin electrode blanks,each having an imperforate portion, are first permanently mounted intheir desired predetermined spaced relationship suitable forincorporation in. an electron tube. Aligned apertures are then cutthrough the imperforate portions of all the blanks in asingle.continuous operation by a method which involves substantially nodeformative force on the blanks.

The requirement of a non-deformative cutting operation results from thedesire to use the improved method in forming apertures in thin, fragile,electrode blanks. For example, the thickness of the walls of the blanksmay be of the order of a few thousandths of an. inch. A thin walledblank is desired because of the resultingability to achieve a mutuallyclose spacing between all portions of the electrodes so formed. Such aclosespacing results in better high frequency operation. Suitable waysof performing the cutting depend upon the rigidity of the blanks to becut. The thinner the blanks, the less the permissible force thereon.Cutting methods which are suitable include: ultrasonic machining,wherein. a,

shaped tool is vibrated at an ultrasonic frequency toward. and away fromthe work while a thin film of abrasive;material suspended in a liquidvehicle is maintained between the tool and the work; electricaldischarge machining,- wherein an electrical discharge is repeatedlyestablished between a shaped tool and the work toerode away the workover an area corresponding to the shape of thetool; and abrasive spray,whereinv abrasive material is. sprayed, e.g. in air, against the workover a predetermined area to cut away the work over that area. Actually,if the blanks to be cut were thick and rigid enough, then an ordinarymilling operation might be used. However, this is not the case in themanufacture of thin electron tube parts to which this invention relates.

In the drawings:

Fig. 1 is an elevation view in partial cross-section of an electron tubemade according to our invention;

Fig. 2 is an enlarged plan view in cross-section of a portion of thetube of Fig. 1 taken along line 2-2 of Fig. 1;

Fig. 3 is an elevation view in partial cross-section of a grid blankassembly'being formed in accordance with, our invention by one method ofcutting;

Fig. 4 is an enlarged plan view of the grid assembly and cutting tool ofFig. 3 taken along line 44 of Fig. 3;,

Fig. 5 is a longitudinal cross-section view of a jig being, used in thepractice of our invention;

Figs. 6 and 7 are views similar to those of Figs. 3 and 4 respectivelyexcept a modified method of forming, the grid apparatus by anothercutting method is shown;

Figs. 8 and 9 are elevation and enlarged plan views in partial crosssection respectively of the cathode and grid assembly of anotherelectron tube made according to our invention; and

Figs. 10 and 11 are elevation and enlarged plan views in partialcross-section respectively of another modified method of forming thegrid apertures according to our. invention.

Referring to Fig. 1, an electron tube 10 comprises a cylindrical cathode12, a control grid 14, a screen grid 16, and an anode 18, all arrangedin nested coaxial array, The cathode 12 includes a tubular sleeve memberhaving at one end a cylindrical electron activeportion 20 coated with anelectron emissive material, and at the other end a radial flange portion22 having at the outer periphery thereof an axially extending tubularportion '24. The ifiange portion 22 and, the tubular portion 24 areadapted to be seated in a cathode mounting cup 26 so that the cathodemay be inserted in the tube in predetermined f a "menace spacedrelationship to the control grid 14 after assembly of the vacuumenvelope portions of the tube. The control grid 14 and the screen grid16 comprise, respectively, electron active cylindrical perforatedportions 28 and 30, conical tubular support portions 32 and 34, radialflange portions 36 and 38, and outer tubular terminal contact portions40 and 42. The anode 18 comprisesa cup-like member disposed over, andsurrounding, the cathode and grids. A metal tubulation 43, brazed to thetop of the anode 18, communicates to within the anode and serves as ameans of evacuating the tube when its fabrication is completed. Afterevacuation the tubulation 43 is pinched off by a cold weld operationtoseal the tube 10. A heater element is provided by a coil 44 of wiredisposed inside the cathode sleeve adjacent the sleeves coatedcylindrical portion 20. One end of the coil 44 is connected to thecathode sleeve 12 and the other end to a heater lead-in assembly 46which comprises a heavy conductor 48 and a terminal cup ,50 disposedcoaxially within the cathode sleeve. The heater lead-in assembly 46,cathode 12, control grid 14, screen grid 16, and anode 18 are all heldin a predetermined insulative spaced relationship to each other in theorder named by a series of separating annular ceramic disk 52-58 sealedrespectively to the radial flange portions of those members. A heatradiator 60, comprising a cylindrical member 62. and a plurality ofannular disk flanges 64 attached thereto, is fitted tightly over theexternal cylindrical surface of the anode.

As an example, the electron tube 10 of Fig. 1 has been fabricated withan overall length of approximately 1.9 inches and with approximate gridand cathode data as follows: an outside diameter of 0.322 inch and anaxial length of 0.435 inch for the electron active portion 28 of thecontrol grid; an outside diameter of 0.338 inch and an axial length of0.425 inch for the electron active portion 30 of the screen grid; 72grid wires of about 0.004 inch square cross-section for each grid; andan outside diameter of 0.290 for the coated portion 20 of the cathode.

Electrons emitted from the coated cylindrical portion 20 of the cathodetravel in a generally radial direction to the cylindrical surface of theanode, passing through aligned perforations, or apertures, in thecontrol grid and the screen grid. In order to obtain the highestpossible operating efiiciecny from the tube, screen grid current must bekept to a minimum. As is well known in the art, this can best beachieved by obtaining an accurate alignment of the conductive elementsof the tube grids so that electrons which pass through the control gridapertures will be presented with corresponding openings in the screengrid rather than a conductive element upon which they may impinge andresult in screen grid current. Fig. 2 best shows this alignment betweenthe elec: tron active wire portion 28of the control grid 14 and theelectron active wire portion 30 of the screen grid 16.

In Fig. 3, according to our invention offlfirst mounting imperforategrid blanks in their ultimate spaced relation to each other and thenforming sets of substantially prefectly aligned apertures therein, theimperforate grid blanks 14a and 16a comprise solid one piece metalmembers shaped identical to the finished grids 14 and 16 shown in Figs.1 and 2 but having solid imperforate cylindrical portions 28a and 30ainstead of the perforate portions 28 and 30 of the finished grids. Asassembly consisting of at least an imperforate control gridblank 14a, animperforate screen grid blank 16a, and a separating annular ceramic disk56 is first fabricated to provide a a series of aligned longitudinalslots equally spaced around the peripheries of the two grid blanks isshown and described. Such a cutting results in a pair of grids having anequal number of longitudinal wires disposed in cylindrical array withthe wires of one grid in substantially perfect radial alignment with thecorresponding wires of the other grid. Fig. 3 shows the cuttingoperation of a grid assembly approximately one-half completed with thecutting being performed longitudinally of the axis of the grids from topto bottom.

An ultrasonic metal cutting (or grinding) machine is provided whichincludes a pan 68 in which the work is A positioned, a vibrationvelocity transformer 72 onto which a predetermined shaped cutting tool74 is attached, and a transducer 76 for vibrating the transformer andtool. The transducer 76, transformer 72, and cutting tool 74, arefixedly mounted above the pan 68. A pan support and, vertical feedmechanism (not shown) below the pan serve to advance the pan work piececontained therein upward toward the tool 74. A magnetic chuck 78 isfixedly attached to the bottom of the pan 68 for securely holding thework in place during cutting.

To produce a finished pair of grids, a grid blank assembly 66 is firsttightly seated in a jigging socket 80. The

. chuck 78 such that the grid blank assembly 66 is axially aligned withthe tool 74 and both may be fitted into their respective recesses of thejig 82. The magnetic chuck 78 is then energized to hold the jiggingsocket 80 and grid blank assembly 66 in proper position for cutting. The

., tool 74 is raised, and the jig 80 is removed. The tool 74 may then beadvanced for cutting.

In operation, an abrasive material is suspended in a liquid vehicle andflowed over the cutting area of the work such as by hoses (not shown)played thereon. At the same time the transducer 76 is repeatedlyenergized to vibrate the tool 74 in a vertical direction, asillustrated,

toward and away from the work. As a result, as the vibrating tool andthe work, i.e., the grid blanks 14a and 1612 are brought close to eachother, the work is worn away over an area directly oppositely andcorresponding to the shape of the tool.

. According to a preferred embodiment of grid structure and a preferredmethod of cutting that structure, the tool 74 comprises a thick walledhollow cylinder having longitudinal teeth 90 internally thereof. Thetool 74 is shown more clearly in cross-section in Fig. 4. The diameterof the tool 74 at the base of the teeth 90 is slightly larger than theoutside diameter of the outer grid blank 16a of the grid blank assembly66. The

a teeth 90 of the tool 74 are sufiiciently long to grid blank assembly66. Because of subsequent assembly extend radially inward to a diametersmaller than the inside diameter of the inner grid blank 14a of the gridblank assembly. As such, when the tool 74 is vibrated and then broughtclose to the grid blanks and advanced downwardly over the grid blanks asshown in Fig. 3, an equal multiplicity of longitudinal slots is cut ineach of the grids in such fashion that a slot in the control grid 14 isas perfectly aligned with a corresponding slot in the screen grid 16 asthe particular tooth of the tool 74 which out those slots is uniformlytrue throughout its length. In effect, this amounts to a substantiallyperfect alignment.

According to a preferred practice of our invention as shown in Figs. 3and 4, the control grid wires 28 and the screen grid wires 30 are thesame width; and the screen grid slots, or apertures, are centrally,radially aligned with, but larger than, the control grid apertures. Sucha structure is preferred over a grid structure having equal widthcontrol grid and screemgrid-apertures because it provides bettershielding of the screen grid wires by the control grid wires andthus-results in less-electron impingement on the screen grid. Anotheradvantage of such a grid design obtains from the shape of the tool teeth90 necessary to out such a grid, and the'shape of a broaching toolsuitable for producing such va tool 74. Since the tool teeth 90 adapted,for forming such a grid assembly are tapered, being thicker at theirbase portion than at their-[end portion, a stronger. toothed tool isprovided. Also a broach suitable" for forming such a tool 76 hasparallel-sided teeth which results in a stronger broach than would bethe casewere the broach teeth of reverse taper, beingv thicker at theirends than at their bases.

Figs. 6 and 7 describe an alternative method of prac-,

ticingour invention. The method ofFigs; 6 and 7 differs from the methodof Figs.-3 and 4 in that the cutting is performed by electricaldischarge machining and in. that a cylindrical tool 92 with externalteeth 94 rather-than a hollow cylindrical tool 74 with internal teeth 90is used.

In cutting by electrical discharge machining. a tank 95' in which theWork'is positioned is filled to a level above the cuttingarea of theworkwith a dielectric oil 96. The tool 92 is supported in a tool holderp97which'is fed downward,'as illustrated, from a feed mechanism (not shown)mounted above the tank 95. A pulsating DC. voltage 98-is applied betweenthe tool .92 and. the grid blanks 14a and 16a by connecting between thetool holder 97 and a grid blank assembly clamp 99. The. clamp 99 servesto maintain the grid blank assembly 100 in fixed position during thecutting operation. When the tool is brought close to the work, a seriesof electrical discharges occur which cause the work piece. i.e., thegrid blanks 14a and 16a to be vaporized or eroded away over an areadirectly oppositeand corresponding to the shape of the tool.

In contrasting the method of Figs. 6 and 7 with that of Figs. 3 and4,the too] is advanced from the opposite end of the grid blank assemblysuch that the tool moves inside the control grid blank 14a rather thandown over the screen grid blank 16a. The diameter of the tool 92 at thebase of the teeth 94 is smaller than the inside diameter of the controlgrid blank 14a; and the diameter of the tool 92 at the outer ends of theteeth 94 is larger than the outside diameter of the screen grid blank16a. This is shown in Fig. 7 where the tool 92 has already advanced topartially form the electron active wire portions 28 and 30 of thecontrol grid 14 and screen grid 16 respectively. As such, a longitudinaladvance of the tool 92.to within the control grid blank 14a produces aseries of substantially perfectly aligned longitudinal slots in bothgrids somewhat similar to that produced by the method of Figs. 3 and 4.

Since the tool 92 must pass through the central openings of all membersof the grid blank assembly 100, the assembly is fabricated to includeonly the two grid blanks 14a and 16a and the single separating ceramicdisk 54. These parts must be dimensioned to accommodate the tool. Thismay necessitate a slightly dififerent shaped supporting portion 32 ofthe control grid blank 14a. But, on the other hand, this method findsparticular advantages in the fabrication of a grid assembly for adouble-ended tube. The term double-ended is meant to describe a gridhaving a tubular supporting portion and a radial flange portion on bothends of a central electron active wire portion rather than having onecupshaped or closed end. Such a structure is known in the art and isparticularly adapted for high frequency operation.

It will be appreciated that the electrical discharge cutting method canbe used with a cylindrical tool having internal teeth such as isdescribed in reference to Figs. 3 and 4. Likewise the ultrasonic cuttingmethod can be used man-a cylindricai toolhaving external teeth H tion toan electron tube similar to the tube of Figs. 1

and 2but-with the" addition of an apertured beam-former shield electrode101 around the coated portion 20 of the cathode sleeve 12. Suchanaddition could be provided for thetube 10 of Figs; 1' and 2 by amodification of the cathodemountingcup 26. ,Fig. .8 illustrates such amodification. The modified mounting cup, constituting the aperturedshield electrode "101, includes an elongated tubular portion-102attached to or integralwith a cup portion-104, which: is identical tothe cathode mounting cup26 of Fig; 1. As "such, there is provided inaddition to the imperforate cylindrical portions 28a and 30a of the twogrid blanksofthe grid blank assembly, a third imperforatecylindrical:member 102 insulatively mounted coaxial therewith. The tubular portion102 is adapted to fit closely around the coated portion 20 of thecathode 12; s

By using aitool similar to the tool 72 of Figs. 3 and 4 but withslightly longer-teeth, aligned slots can be cut through not only theimperforate portions 28a and 30a of the control grid and screen gridblanks but also through the imperforatet. tubular portion 102 to formthe apertured shieldl 98.. Such. a structure serves to permit a copiousemission of electrons from the cathode over only those areas bounded bythetslots or apertures in the shield member 985 Thus, electrons arebeamed toward the anode through the. aligned. grid apertures rather thanbeing emitted,- at. all points around the cathode and being attracteddirectly toward the grid wires as well as through the grid apertures.This even-more elfectively reduces electron interception by the grids,particularly by the control grid.

We havedesoribed= the preferred practice of our invention;as. appliedto. the; cutting of. elongated, longitudinal slots insa series of spacedcylindrical walls by the axial advance of acylindrical tool having aplurality of longitudinal teeth However,-a variety of modifications ofmy invention: arepossible.

For, example, referring to Figs; 10 and 11,. aligned apertures could becut in the cylindrical grid blank assembly 66-by cutting one set ofradially aligned aperturesat a time.- A- single toothed tool 1061sadvanced radiallythrough the imperforatewall portions 30a and ZSa-ofthe' grid blanks-16a and14a, then the tool vvith drawn,- the grid blanleassembly- 66 angula-rlyrotated and indexed, and another set of aperturescut. This procedure would be repeated until aligned apertures wereprovided completely around the periphery of the grid blanks. In Fig. 11a number of such sets of aligned apertures are shown as already cut withthe tool not yet Withdrawn from the cutting of one set.

According to this method of cutting, any shape apertures could be cut,e.g. circular, square, or even circumterential slots part way around theperiphery.

Another modification of our invention resides in the generalconfiguration of the electrodes themselves. The imperforate portions ofthe electrode blanks need not be cylindrical. A planar electrodeassembly can be provided with apertures by the same method but with theuse of a flat tool having one or more elongated teeth, each of whichcuts through all of the planar blanks of the electrode assembly.

These and other modifications of the broad concept of our invention,such as electrode shape and use, aperture shape, cutting method, toolconfiguration and the like will be readily suggested to one skilled inthe art of electron tube fabrication.

It should be apparent from a reading of the description of ourinvention, that several alignment problems hereinbefore mentioned, whichexist in the prior art, are solved according to our invention. It hasbeen earlier stated herein that in order to chain a reasonable alignmentof, for example, cylindrical grids" according to priorart methodsof tubefabrication: (l) the control grid and screen grid units must befabricated with identical configurations but with'the screen gridsomewhat larger, (2) that they must be coaxially'rnounted with neithershifted nor canted axes; and (3) that they must be oriented with respectto each other. According to our invention 'the third of theserequirements is achieved to perfection by a cutting ofapertures with asingle tool subsequent to mounting of grid blanks in their ultimatespaced relation. And, while we do not claim that our invention providesa way of achieving the first two of these requirements, it does evenbetter in that these requirements cease to exist insofar as aperturealignment is concerned when tubes are fabricated according to ourinvention. Even if the grid blanks 14aand 16a are not made with theirimperforate portions 28a'and30a identicaliin configuratiom'and even ifthey are not mounted exactly coaxially, nevertheless, when the:apertures are formed according to our invention they are still inperfect radial alignmenL' Thus, insofar as aperture alignment isconcerned, our invention provides an essentially perfect solution to theproblems of the prior art.

. What is claimed is:

a 1. The method of fabricating an electron tube subassembly comprisingthe steps of first mounting in fixed relation to each other a pluralityof relatively thin electrode blanks having imperforate longitudinalportions in parallel, facing and spaced relationship and in registeralong a line passing through their thin dimensions, and then cuttingapertures simultaneously in said imperforate portions of said blanks in'a direction normal to said line and longitudinally of said blanks in asingle continuous operation by successively removing particles from saidportions of such small size as to be substantially free from exerting adeformative force an said blanks.

2. The method of fabricating an electron tube subassembly, comprisingthe steps of first mounting in a concentric array and in a radiallyspaced relation a plurality of relatively thin walled hollow cylindricalelec trode blanks to dispose imperforate portions thereof in radialregister, fixing said blanks in said spaced array, and thensimultaneously cutting at least one set of radiallyaligned apertures inboth of said electrode blanks in a single continuous operation and in adirection parallel to the axis of said array by eroding from saidimperforate portions relatively small particles only, of the material ofsaid portions, whereby said blanks are substantially free of anydeformation force during said apertureforming step. I

'3. The method of fabricating an electron tube sub-assembly, comprisingthe steps offirst mounting a plurality of relatively thin walledimperforate hollow cylindrical electrode blanksin concentric array,mechanically connecting saidblanks to each other in insulated and fixedrelation, and thencutting a plurality of sets of radially-alignedaxially-extending slots uniformally spaced around the periphery of saidblanks in a direction parallel'tothe axis of said array and in a singlecontinuous operation, while'substantially refraining from exerting adeformative forceon said grid blanks.

4. The method of fabricating an electron tube subassemb'ly comprisingthe steps of first mounting a plurality of electrode blanks, each havinga relatively thin imperforate hollow cylindrical wall portion, inpermanent insulative spaced relationship with their said imperforatewall portions coaxial. and coextensive, then cutting a multiplicity ofsets of radially aligned, longitudinally extending slots in saidelectrode blanks, by simultaneously eroding a plurality oflongitudinally extending portions of said wall portions in'a directionaxially of said wall portions. 5. The method according to claim 4wherein the slot cutting step isaccomplished by ultrasonic machining.

6. Method of fabricating a sub-assembly for an electron tube comprisingthe steps of first mounting in fixed, spaced and insulated relation, aplurality of electrode blanks having thin spaced wall portions, toprovide a sub-assembly .wherein a predetermined plane intersects saidwall portions; and then cutting one set of apertures through saidportions and in said plane by advancing an erosion -zone,through.saidportions and in said plane while preservingsaid portions from contactwith material of sufficient mass to deform said portions.

. References Cited in the file of this patent UNITED STATES PATENTS 3 IOTHER' REFERENCES 1 Machinery, published April 1954, page 244.

